RSSI is a measure of the signal strength between transmitting and receiving devices. RSSI is a measure of the strength (not necessarily the quality) of the received signal in a radio frequency environment.
RSSI is measured in arbitrary units. RSSI can be used internally in a radio frequency networking card to determine when the signal is below a certain threshold. This allows determination of the type of messages to send as well as the timing for those messages.
A radio frequency network monitoring tool can be used to measure the signal strength of a wireless network. From this a RSSI value can be determined. An end-user can determine an RSSI value when measuring the signal strength of a wireless network on a radio frequency network monitoring tool.
RSSI values are used in a range of situations including the transmission of signals over wires and in various radio frequency technologies including the General Packet Radio Service (GPRS), Global System for Mobile Communications (GSM), Code division multiple access (CDMA), Bluetooth™, ZigBee™, Wifi 802.11x, and other Radio Frequency Identification (RFID) methods.
The value of RSSI measurements will vary depending on the implementation and the chosen scale, but is usually an integer value where a low value indicates a low signal strength. According to the IEEE802.11 standard:                RSSI is intended to be used in a relative manner. Absolute accuracy of the RSSI reading is not specified.        
When transmitting messages between devices the RSSI is a useful guide to the strength of the signal whether measured in Watts (W) or Decibels (dB).
While other factors come into play, distance has the greatest impact on signal strength in a radio frequency environment. In general, the strength of the signal (ss) is understood to be proportional to the inverse square of the distance (d):ssα1/d2 
Signal strength (ss) and RSSI can be used as a guide to the distance between transmitting and receiving radio frequency devices.
RSSI is used in different ways depending on what the primary objective is.
When the transmission of messages is the aim, the relative distance of the transmitting and receiving devices is not important. Only the strength and quality of the signal is important in this case.
Where the proximity of two objects is the key criterion, for example with location based services or security applications, the strength of the signal is not important. However, the relative physical locations of the objects will be important.
In each case, fluctuations in the RSSI are prevalent. These are due to various reasons including changes in distance, interference from external material such as wood or metal between the radio frequency devices, and environmental conditions.
In order to more fully exploit the use of radio frequency devices in the provision of location based services or security applications a need exists to determine the distance between such devices or relative change of distance between such devices, given the presence of these fluctuations.
The provision of location based services or security applications requires the operation of switches. Various means and mechanisms for remotely controlling switches are known. The remote control of switches can be used to allow or restrict access to buildings or premises. In addition, remote control devices are well known for locking and unlocking cars, garages and the like. Further, remote control devices can be used in combination with electronic devices such as alarm systems, lights, televisions, stereos and DVD players.
Remote controls typically require some kind of action on the part of a user to operate them. There may be a button which a user has to depress in order to obtain operation of the device, for example, the unlocking of a door. Alternately, the user may be required to put a key within a lock. These requirements can be inconvenient if the user has already got their hands full, for example, if they are carrying bags or other items or if a user is already operating another device, for example, driving a car.
Systems have been proposed which utilise the radio frequency technology Bluetooth™ to allow operation of a switch. However, typically these systems require the device to be discoverable (visible to inquiry) and/or to connect to a specific application on a specific profile. An example of such a system is described in the specification accompanying international application no. PCT/SE01/00285 (publication no. WO 01/63425).
Wide scale utilisation of the radio frequency technology Bluetooth™ in the system described in WO 01/63425 has limitations as many Bluetooth devices are not permanently discoverable. Action is required on the part of the user to change the status of the device to make it discoverable.
It is an object of the invention to provide an improved method for determining the proximity or relative changes in proximity between radio frequency devices.
It is an object of the invention to provide a method for controlling a switch based on the proximity or relative changes in proximity between radio frequency devices.
These objects are to be read disjunctively with the object to at least provide the public with a useful choice.